An I.S. machine has a plurality of identical sections each of which has a blank station which receives one or more gobs of molten glass and forms them into parisons and a blow station which receives the parisons and forms them into bottles. The blow station includes a mold, including a pair of opposed side molds. The side molds are first separated to an open position so that a formed parison can be located in the blow station. The side molds are then displaced to the closed position clamping a mold bottom in the process. A blowhead is then located on top of the closed side molds to close the mold and when the parison has completed “reheat,” the blowhead blows the parison into a bottle. A number of small holes are defined in the mold so that a vacuum can be applied between the outer wall of the parison and the mold surface. As the parison is blown, the vacuum draws the air, in this decreasing volume, out of the mold. In a conventional vacuum valve, a constant vacuum is applied.